In a synchronous motor with a permanent-magnet field system, a rotor which includes a plurality of permanent magnets arranged at generally equal intervals around a shaft, and a plurality of core members arranged around the shaft so as to hold each magnet between the core members in a circumferential direction, has already been known. In this arrangement, the permanent magnets are magnetized in a circumferential direction, whereby each of the core members forms a magnetic pole. In such a conventional rotor, each permanent magnet is generally brought into close contact with the side surface of each adjacent core member, and is generally fixedly supported in a radial direction by outer and inner hooking protrusions which are formed respectively at axial outer and inner peripheries of side surface of each core member. On the other hand, each core member formed by generally stacking a plurality of magnetic materials, such as silicone steel plates, includes an axially extending through hole at a generally center thereof. Rod members are inserted into respective through holes. Annular end plates are arranged in close contact with axial end surfaces of the permanent magnets and the core members. Each annular end plate is fixed to the shaft at a center hole thereof, and has openings arranged around the center hole and aligned with the through holes of the core members. The rod members pass through the openings of each end plate and are fixed to the end plate at the both ends of the rod members, the ends projecting from the axial end surfaces of the core members. In this manner, the permanent magnets and the core members are fixedly supported in a rotor construction against an external force, such as a centrifugal force, through the 10 end plates, the rod members, and the hooking protrusions.
The radially outer surfaces of the core members confront a stator core with a small gap therebetween, and the main magnetic flux generated by the permanent magnets extends through the core members and the gap to the stator core. Ideally the magnetic flux distribution in the gap established between the core members and the stator core draws a sine curve with apexes at centers of the radially outer surfaces of the respective core members. In this ideal condition, the torque fluctuation of the motor can be substantially eliminated. Consequently, this kind of rotor enables the distribution of the main magnetic flux in the gap to easily approximate the ideal sine curve, by shaping the radially outer surfaces of the core members into surfaces curved under given rules.
The distribution curve of the main magnetic flux is, however, deviated from the predetermined optimum sine curve of the magnetic flux distribution, due to an armature-reaction caused by a winding current which passes through a winding on a stator when the rotor rotates, whereby a torque ripple is generated in the motor. This is considerable particularly when the winding has numbers of turns or the large current is passed through the winding. FIGS. 6 and 7 diagrammatically show a condition when the distribution of the main magnetic flux is deviated due to the armature-reaction. As shown in FIG. 7, an armature-reaction magnetic flux F.sub.r generally has a phase difference of 90.sup..degree. from a main magnetic flux F. Also, as shown in FIG. 6, the core members C can easily constitute magnetic paths of the armature-reaction magnetic flux F.sup.r, so that the magnetic paths of the main magnetic flux F in the core members are deflected in a circumferential direction due to the armature-reaction magnetic flux F.sup.r, whereby the deviation is caused in the distribution curve of the main magnetic flux F. As shown in FIG. 7, the distribution of a magnetic flux composed of the main magnetic flux F and the armature-reaction magnetic flux F.sub.r is remarkably deviated from the main magnetic flux F. Further, when the main magnetic flux F is deflected in the circumferential direction due to the armature-reaction, a local magnetic saturation is caused in the core members C, whereby the peak torque of the motor is deteriorated.
As mentioned above, the main magnetic flux, which should form symmetrical magnetic paths relative to the symmetrical plane of the core member, the plane axially extending while involving a center axis, so as to obtain the magnetic flux distribution based on the ideal sine curve between the core member and the stator core, is deflected in the symmetrical form of the magnetic paths thereof due to, e.g., the armature-reaction, whereby the local magnetic saturation is caused in the core members. Such a resulting asymmetrical state of the magnetic paths may be caused by various factors other than the armature-reaction. The local magnetic saturation in the core members brings about problems wherein, e.g., the peak torque of the motor is deteriorated, the smooth rotation of the rotor is disturbed, the current control becomes de-stabilized, the heat generation is increased by the increase in iron loss.